I have all the systems:
The Wii gives you the best in terms of getting the casual gamer to play; plus it's a lot of fun
The Xbox 360 is the best gaming system IMO. The Kinect is fun as your whole body is the controller
The PS3 has pretty much the same games as the Xbox 360; the PS3 has a blu-ray player which is a very nice feature if you love everything HD.
Good luck!